[C++20] [Modules] Prevent to accept clang modules

Close #64755

This wouldn't affect the form @import as the test shows. The two
affected test case `diag-flags.cpp` and `diag-pragma.cpp` are old test
cases in 2017 and 2018, when we're not so clear about the direction of
modules. And the things that these 2 tests tested can be covered by
clang modules naturally. So I change the them into clang modules to
not block this patch.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -11318,6 +11318,8 @@ def err_module_import_in_implementation : Error<
   "@import of module '%0' in implementation of '%1'; use #import">;
 
 // C++ Modules
+def err_module_import_non_interface_nor_parition : Error<
+  "import of module '%0' imported non C++20 importable modules">;
 def err_module_decl_not_at_start : Error<
   "module declaration must occur at the start of the translation unit">;
 def note_global_module_introducer_missing : Note<

clang/lib/Sema/SemaModule.cpp

@@ -531,6 +531,12 @@ DeclResult Sema::ActOnModuleImport(SourceLocation StartLoc,
   if (!Mod)
     return true;
 
+  if (!Mod->isInterfaceOrPartition() && !ModuleName.empty()) {
+    Diag(ImportLoc, diag::err_module_import_non_interface_nor_parition)
+        << ModuleName;
+    return true;
+  }
+
   return ActOnModuleImport(StartLoc, ExportLoc, ImportLoc, Mod, Path);
 }
 

clang/test/Modules/cxx20-export-import.cpp

@@ -1,4 +1,14 @@
 
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -std=c++20 -fmodules -fmodules-cache-path=%t -fimplicit-module-maps -I%S/Inputs -verify %s
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/dummy.cppm -emit-module-interface -o %t/dummy.pcm
+// RUN: %clang_cc1 -std=c++20 -fprebuilt-module-path=%t -verify %t/test.cpp
+
+
+//--- dummy.cppm
+export module dummy;
+
+//--- test.cpp
 export import dummy; // expected-error {{export declaration can only be used within a module purview}}

clang/test/Modules/diag-flags.cpp

@@ -30,7 +30,7 @@
 // RUN: %clang_cc1 -triple %itanium_abi_triple -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -std=c++20 -DERROR -fmodule-file=%t/werror.pcm -Wno-error
 // RUN: %clang_cc1 -triple %itanium_abi_triple -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -std=c++20 -DERROR -fmodule-file=%t/werror.pcm -Wno-padded
 
-import diag_flags;
+#include "diag_flags.h"
 
 // Diagnostic flags from the module user make no difference to diagnostics
 // emitted within the module when using an explicitly-loaded module.

clang/test/Modules/diag-pragma.cpp

@@ -4,7 +4,7 @@
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -emit-module -fmodule-name=diag_pragma -x c++ %S/Inputs/module.map -std=c++20 -o %t/explicit.pcm -Werror=string-plus-int
 // RUN: %clang_cc1 -fmodules -fimplicit-module-maps -verify -fmodules-cache-path=%t -I %S/Inputs %s -std=c++20 -DEXPLICIT_FLAG -fmodule-file=%t/explicit.pcm
 
-import diag_pragma;
+#include "diag_pragma.h"
 
 int foo(int x) {
   // Diagnostics from templates in the module follow the diagnostic state from
@@ -42,7 +42,6 @@ int foo(int x) {
 
   if (x = DIAG_PRAGMA_MACRO) // expected-warning {{using the result of an assignment as a condition without parentheses}} \
                              // expected-note {{place parentheses}} expected-note {{use '=='}}
-                             // expected-error@-2 {{use of undeclared identifier 'DIAG_PRAGMA_MACRO'}}
     return 0;
   return 1;
 }

clang/test/Modules/pr64755.cppm

@@ -0,0 +1,17 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I%t -fmodule-name=a0 -x c++ -emit-module %t/module.modulemap -o %t/a0.pcm
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fmodule-file=%t/a0.pcm -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fmodule-file=a0=%t/a0.pcm -verify -fsyntax-only
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp -fprebuilt-module-path=%t -verify -fsyntax-only
+
+//--- module.modulemap
+module a0 { header "a0.h" export * }
+
+//--- a0.h
+void a0() {}
+
+//--- use.cpp
+import a0; // expected-error {{import of module 'a0' imported non C++20 importable modules}}